Many modern engines and next generation turbine engines require components and parts having intricate and complex geometries, which require new types of materials and manufacturing techniques. Conventional techniques for manufacturing engine parts and components involve the laborious process of investment or lost-wax casting. One example of investment casting involves the manufacture of a typical rotor blade used in a gas turbine engine. A turbine blade typically includes hollow airfoils that have radial channels extending along the span of a blade having at least one or more inlets for receiving pressurized cooling air during operation in the engine. Among the various cooling passages in the blades, includes serpentine channel disposed in the middle of the airfoil between the leading and trailing edges. The airfoil typically includes inlets extending through the blade for receiving pressurized cooling air, which include local features such as short turbulator ribs or pins for increasing the heat transfer between the heated sidewalls of the airfoil and the internal cooling air.
The manufacture of these turbine blades, typically from high strength, superalloy metal materials, involves numerous steps. First, a precision ceramic core is manufactured to conform to the intricate cooling passages desired inside the turbine blade. A precision die or mold is also created which defines the precise 3-D external surface of the turbine blade including its airfoil, platform, and integral dovetail. The ceramic core is assembled inside two die halves which form a space or void therebetween that defines the resulting metal portions of the blade. Wax is injected into the assembled dies to fill the void and surround the ceramic core encapsulated therein. The two die halves are split apart and removed from the molded wax. The molded wax has the precise configuration of the desired blade and is then coated with a ceramic material to form a surrounding ceramic shell. Then, the wax is melted and removed from the shell leaving a corresponding void or space between the ceramic shell and the internal ceramic core. Molten superalloy metal is then poured into the shell to fill the void therein and again encapsulate the ceramic core contained in the shell. The molten metal is cooled and solidifies, and then the external shell and internal core are suitably removed leaving behind the desired metallic turbine blade in which the internal cooling passages are found.
The cast turbine blade may then undergo additional post-casting modifications, such as but not limited to drilling of suitable rows of film cooling holes through the sidewalls of the airfoil as desired for providing outlets for the internally channeled cooling air which then forms a protective cooling air film or blanket over the external surface of the airfoil during operation in the gas turbine engine. However, these post-casting modifications are limited and given the ever increasing complexity of turbine engines and the recognized efficiencies of certain cooling circuits inside turbine blades, the requirements for more complicated and intricate internal geometries is required. Moreover, as internal geometries become more intricate, additional machining needs to be aligned with the internal features. For example, the cooling holes drilled through the sidewalls of the airfoil should align with internal air passages.
In conventional methods, a cast part includes external cast datums formed in the exterior surface of the part by the casting shell. The part is loaded into a fixture that constrains the part against the cast datums. The part is then machined based on a three-dimensional model of the part (e.g., a computer-aided design (CAD) model). The present inventors have discovered that in some cases, features formed by the casting core may be offset from the cast datums due to core shift that occurs in production of the internal cast features. Accordingly, machining based on the external datums using a nominal CAD geometry may be difficult or inaccurate. Accordingly, it is desired to provide an improved casting method for three dimensional components having intricate internal voids.